Transformer core with adjustable airgap

ABSTRACT

A transformer core made up of two sections separated by an airgap, the width of this gap being adjustable to minimize the overall transformer loss ratio. A pair of small runner tubes are affixed to one of the core sections, each tube being internally threaded to receive a screw journaled on the other core section. Rotation of these screws allows for precise adjustment of the width of the gap between the core sections to a point where maximum power transfer to an output circuit is obtained.

United States Patent [72] lnventors Harry P.Lee

1300 Edgewood Way, Apt. 105, Oxnard, Calif. 93030;

Peter Billic, 1446 W. Beverly Drive, Oxnard, Calif. 93030; Clifford B. Alfors, 4671 Student St., Ventura, Calif. 93003 [21] Appl. No. 50,611 [22] Filed June 29, 1970 [45] Patented Nov. 23, 1971 [54] TRANSFORMER CORE WITH ADJUSTABLE AIRGAP 1 Claim, 3 Drawing Figs.

52 0.5. CI 336/134 [51] lnt.Cl ..1101f21/06 [50] Field of Search 336/130,

[56] References Cited UNITED STATES PATENTS 2,523,117 9/1950 Jennings 336/134 X 1,054,862 3/1913 Paling 336/134 X 2,840,789 6/1958 Miller 336/134 1,852,358 4/1932 Merkel..... 336/134 X 2,315,609 4/1943 Fielder 336/134 X 2,956,250 10/1960 Harse 336/210 X 3,017,544 l/l962 Kane et al. 336/134 2,437,021 3/1948 Fries 336/134 FOREIGN PATENTS 297,173 5/1954 Switzerland Primary Examiner-Thomas J. Kozma AnorneysRichard S. Sciascia, Q. Baxter Warner and Howard J. Murray, Jr.

ABSTRACT: A transformer core made up of two sections separated by an airgap, the width of this gap being adjustable to minimize the overall transformer loss ratio. A pair of small runner tubes are affixed to one of the core sections, each tube being internally threaded to receive a screw journaled on the other core section. Rotation of these screws allows for precise adjustment of the width of the gap between the core sections to a point where maximum power transfer to an output circuit is obtained.

PATENTEnHuv 2 3 ml HARRY P. LEE

CLIFFORD B. ALFORS PETER B/LL/C INVENTOR 25M 1&-

TRANSFORMER CORE WITH ADJUSTABLE AIRGAP STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for The Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION When an airgap is introduced between the two U-shaped cores of a-standard transformer, the effective permeability of the core material is increased andthe loss ratio (power lost to power stored) is decreased. The former changes the inductance of the windings and hence the overall frequency response, while the latter, which is the sum of the core loss ratio and the winding loss ratio, is at a minimum when the decrease in core loss ratio is not offset by an increase in the lossratio of the windings.

This balancing" of coreflux gain against winding resistance loss to achieve maximum power transfer efficiency necessitates a very precise adjustment of the air gap, since any increase in gap width requires an increase in magnetizing current to produce the same magnetic flux. However, as the current increases, the loss due to the resistance of windings also increases. Thus, gap adjustment is critical in order not to offset core flux gain by winding resistance loss. While this fact is known in the art, present transformer designs do not provide the precise airgap width control needed for optimum operating efficiency.

SUMMARYOF THE INVENTION The present concept is directed to a technique for adjusting the width of an airgap in a transformer core in order to minimize the overall loss ratio and yield maximum power transfer. In a preferred embodiment, a pair 'of small runner tubes are affixed to one of the two U-shaped cores of a standard transformer, each tube being internally threaded to receive a screw journaled on the remaining core section. Independent rotation of these screws allows for precise adjustment of the width of the gap between the core sections to a point where maximum power transfer to an output circuit is obtained.

I STATEMENT OF THE'OEJECTS OF THE INVENTION One object of the present invention, therefore, is to provide means for precisely adjusting the width of an airgap in a transformer core.

Another object of the invention is to provide means for enabling the airgap in a transformer core to be adjusted for maximum power transfer.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic illustration of a transformer core having an airgap therein, the showing'ybeing such as to facilitate an understanding of the principles upon which the present concept is based;

FIG, 2 is an exploded view of a transformer core adjusting assembly designed in accordance with a preferred embodiment of the present invention; and

FIG. 3 is a perspective view of the transformer core adjusting assembly of FIG. 2 when thedevice is ready for operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT In any transformer a certain amount of distributed capacitance is present in the windings. If a step-up transformer is used as an example with a large secondary-to-primary turns ratio, then it can be assumed that practically all of this capacitance is present in the secondary coil. This capacitance constitutes, with the inductance of the coil, a low-Q band-pass filter with a peak frequency response fo. Now, if the operating frequency of the transformer is either far above or far below this natural frequency fo, the efficiency of the device will be low. Power transfer is at a maximum only when the operating frequency coincides with the natural transfonner frequency, any alternation being due solely to resistance of the windings andlloss in the core material per se.

One method of matching the operating frequency to the natural transformer frequency is by providing an airgap in the core and adjusting this gap to modify the inductance L of the windings as a function of changes in core permeability.

The inductance of a coil in a'transformer can be expressed as L=( N I /i l where N is the number of turns,

1' is the current flowing through the coil,

4 is the flux.

The flux I -can be related by ==BA=uHA where Bis the magnetic induction,-

A is the cross-sectional area of the lines of the magneticinduction u is the permeability of the medium enclosed by the coils of the transformer,

H is the magnetic field intensity.

The line integral around a magnetic circuit, as shown in FIG. 1 of the drawings, is I For a typical ferrite core, u is about 2,000 times larger than the u of air. Thus,- a small airgap can have a material effect on inductance. For example, a secondary coil with N turns will have the following inductance change when an air gap is introduced:

Let the length of the air gap be lpercent of the total effective core length I,.,'and let the cross-sectional area of the air gap be 25 percent larger than that of the core A,. Thus, the ratio of the inductance of the secondary coil in a transformer that has an air gap. to one that has no air gap can be expressed as follows: I

.l .11 air IID For this particular example, the inductance is decreased by 17 times when the air gap is increased by 1 percent of the effective magnetic length of the core.

When an air gap is introduced in a transformer, the loss ratios for the core material and the windings are also affected. This effect is shown below:

The total loss ratio T of a transformer can be expressed as the sum of the core loss ratio and the winding loss ratio:

where P is the core loss,

E is the rms value of the induced voltage, I is the rrns core-loss component of the current, 1 is the rms magnetizing component of the current, R is the resistance'of the Winding.

When an air gap is introduced, the magnetizing current must increase in order to maintain the same flux density as when no air gap is present. Hence, the core loss P and the induced voltage E are maintained constant. As can be seen in Eq. (9), as 1,, increases, the reactive power is increased, but the loss ratio due to the core material is decreased. At the same time, the loss due to the resistance in the windings is increased because of the increased current. Consequently, the air gap should be adjusted such that the decrease of loss ratio due to the core is not offset by the increase of loss ratio due to the resistance in the winding.

To determine the value of I,,, that results in the minimum loss ratio, Eq, (9) is differentiated with respect to I,,, and the derivative set equal to zero; thus,

The minimum loss ratio of the transformer is thus obtained when the air gap is adjusted so that the copper loss due to the magnetizing current equals the sum of the core loss and the copper loss due to the core-loss current.

FIGS. 2 and 3 of the drawings illustrate a transformer having an airgap the width of which is precisely adjustable in order to carry into effect the principles set forth above. It is made up of a pair of U-shaped core sections and 12 disposed as shown in the drawing to form a rectangular assembly, the respective legs 14 and 16 of the sections 10 and 12 being separated by an airgap 18, as shown in FIG. 3, and the respective legs 20 and 22 being separated by a further gap 24. The windings have been omitted from the drawing for the sake of simplicity. Each of the legs l4, 16, 20 and 22 has a slot or groove formed therein. the grooves 26 and 28 in the core legs 14 and 16 being aligned with one another to receive a threaded pin 30, and with the grooves 32 and 34 in core legs 20 and 22 being similarly aligned to receive a further threaded pin 36. The grooves 26, 28, 32 and 34 have smooth surfaces designed to permit free rotation of the respective pins 30 and 36 therein when the unit is assembled as shown in FIG. 3 of the drawings. Each pin 30 and 36 has a slot 38 and 40, respectively, formed therein to allow for rotation of the pin by a manually held tool when the width of the airgaps l8 and 24 (FIG. 3) is to be adjusted in a manner to be subsequently described.

Each of the pins 30 and 36 has an upper threaded portion 42 and 44, respectively, lying adjacent the core legs 14 and 20 when the transformer is assembled as shown in FIG. 3. The lower end 46 of pin 30 is of reduced diameter to pass through an opening 48 in a retainer plate 50, where it is locked in place by a pin 52 passing through opening 54. Similarly. the lower end 56 of pin 36 is of reduced diameter to pass through opening 58 in plate 50, where it is locked in place by a pin 60 passing through opening 62. The plate 50 is secured to the lower surface of the center portion of core section 12, as shown in FIG. 3, preferably by means of some adhesive substance such as epoxy resin.

The slot 26 in core leg 14 is designed to receive a runner tube 64 (FIG. 2), and the slot 32 in core leg 20 is designed to receive a similar runner tube 66. Each of these runner tubes 64 and 66 is secured to the inner surface of its respective groove by some adhesive substance such as epoxy resin (indicated in FIG. 3 by the reference numeral 68). The inner surface of runner tube 64 is threaded to receive the threaded portion 42 of pin 30, while the inner surface of runner tube 66 is similarly threaded to receive the threaded portion 44 of pin 36. It will now be seen that the pins 30 and 36 are respectively journaled in openings 48 and S8 of plate 50. Although the latter is secured to the core section 12, rotation of either or both pins 30 and 36 does not produce any positional displacement of this core section.

However, rotation of either or both pins 30 and 36 causes an axial movement of either or both runner tubes 64 and 66, since these tubes are prevented from rotating due to their attachment to core section 10 by the adhesive 68. When the runner tubes 64 and 66 undergo this axial movement, the core section 10 is also displaced relative to core section 12 by a distance dependent upon the amount of rotation of pins 30 and 36. This relative displacement of core sections 10 and 12 increases or decreases the width of the air gaps l8 and 24, depending upon the direction of rotation of the pins 30 and 36, and hence permits the transformer to be regulated for maximum operating efficiency through a balance of core flux gain against winding resistance loss.

It will be recognized that the concept herein disclosed is intended to embrace other electrical components, such as inductors, where an adjustable air gap may be introduced into a flux-carrying member in order to vary the amount of energy transferred to an output circuit.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Iclaim: 1. In an electrical transformer having a ferrite core designed in the form of two generally U-shaped members disposed in spaced-apart confronting relationship so as to establish a pair of airgaps between the diametrically opposed legs of said members, the respective widths of such airgaps being adjustable to vary the overall transformer loss ratio and hence the input-to-output power transfer, each pair of diametrically opposed legs having axially aligned grooves formed therein, the improvement which comprises:

a pair of internally threaded collars having outer dimensions compatible with the inner dimensions of said grooves;

means for respectively securing said pair of collars within the grooves formed in the legs of one of said U-shaped members;

a pair of rods respectively freely receivable in said pair of grooves and passing through said pair of collars, that portion of each rod passing through the groove in said one U- shaped member being threaded to engage the threaded inner surface of the collar through which it passes;

a retainer plate secured to the other of said U-shaped members and lying in a plane normal to the axis of said grooves and of said pair of rods;

said retainer plate having a pair of openings therein through which said pair of rods respectively pass;

each of said pair of rods having both an interior shoulder contiguous with and bearing against one surface of said retainer plate as well as a transverse opening adjacent the other surface of said retainer plate; and

a pair of pins respectively passing through the openings in said pair of rods and serving, with said shoulders, to preclude axial movement of said rods with respect to the other of said U-shaped members.

whereby selective manual rotation of either rod of said pair will result in an axial movement with respect to the said other U-shaped member of the collar through which said selected rod passes, and hence of the core leg to which said collar is secured.

the two said airgaps being thus individually adjustable in width to provide precise vemier control over the loss ratio of said transformer and hence the amount of power transferred from the input to the output thereof. 

1. In an electrical transformer having a ferrite core designed in the form of two generally U-shaped members disposed in spacedapart confronting relationship so as to establish a pair of airgaps between the diametrically opposed legs of said members, the respective widths of such airgaps being adjustable to vary the overall transformer loss ratio and hence the input-to-output power transfer, each pair of diametrically opposed legs having axially aligned grooves formed therein, the improvement which comprises: a pair of internally threaded collars having outer dimensions compatible with the inner dimensions of said grooves; means for respectively securing said pair of collars within the grooves formed in the legs of one of said U-shaped members; a pair of rods respectively freely receivable in said pair of grooves and passing through said pair of collars, that portion of each rod passing through the groove in said one U-shaped member being threaded to engage the threaded inner surface of the collar through which it passes; a retainer plate secured to the other of said U-shaped members and lying in a plane normal to the axis of said grooves and of said pair of rods; said retainer plate having a pair of openings therein through which said pair of rods respectively pass; each of said pair of rods having both an interior shoulder contiguous with and bearing against one surface of said retainer plate as well as a transverse opening adjacent the other surface of said retainer plate; and a pair of pins respectively passing through the openings in said pair of rods and serving, with said shoulders, to preclude axial movement of said rods with respect to the other of said U-shaped members, whereby selective manual rotation of either rod of said pair will result in an axial movement with respect to the said other U-shaped member of the collar through which said selected rod passes, and hence of the core leg to which said collar is secured, the two said airgaps being thus individually adjustable in width to provide precise vernier control over the loss ratio of said transformer and hence the amount of power transferred from the input to the output thereof. 